Much research has been carried out on methods for synthesizing carboxylic acid. Such methods are one of the important reaction technologies in synthetic organic chemistry. Known reactions for oxidizing alcohols, aldehydes and the like include an oxygen oxidation reaction using a solid catalyst; an oxidation reaction using a chemical oxidizer such as chromic acid, potassium permanganate, nitric acid or the like; and a liquid phase autoxidation reaction. These conventional methods, however, have problems such as high costs of solid catalysts, difficulty in selective conversion to carboxylic acid in some cases, and high toxicity of many chemical oxidizers.
Methods are known for oxidizing a fluoroalkyl alcohol to the corresponding fluoroalkylcarboxylic acid with a comparatively high yield. Such methods include methods using a chemical oxidizer, for example, potassium dichromate/sulfuric acid (T. Hudlicky et al., J. Fluorine Chem., (1992), 59(1), 9-14), potassium permanganate (I. Lehms et al., DD 268685), or nitrogen dioxide (R. M. Scribner, J. Org. Chem., (1964), vol.29, 279-283 or ibid., (1964), vol.29, 284-286); and methods using an organic or inorganic acid copper salt catalyst/alkali/oxygen (I. P. Skibida et al., WO 93/12059). These methods, however, have the following problems. The method using chromic acid or potassium permanganate has waste disposal problems after reaction. According to the method using nitrogen dioxide, the reaction takes 10 hours or more using 2 equivalents of nitrogen dioxide. Moreover, use of an increased amount of nitrogen dioxide and/or a higher reaction temperature will increase byproducts. The oxidation method using an organic or inorganic acid copper salt catalyst/alkali/oxygen only achieves a low selectivity to carboxylic acid and also has separation and purification problems such as difficulty in removal of the catalyst and the solvent after reaction.
Methods for oxidizing a fluoroalkyl alcohol to the corresponding fluoroalkylcarboxylic acid using nitric acid are described, for example, in D. R. Bear, Ind. Eng. Chem., (1959), vol.51, 829-830 and in Y. Desirant, Bull. Sci. acad. roy. Belg., (1929), vol.15, 966-982. However, Desirant reports that the method has the following problems: the reaction requires using about 2.5 equivalents of nitric acid relative to the alcohol and it takes 2.5 days under reflux to complete the reaction. In hydrocarbon oxidation using nitric acid, it is known that oxygen is introduced into the reaction system to reduce the required amount of nitric acid to a stoichiometric amount or less relative to the reaction substrate (John W. Ager, Jr. DE 2116212). Such technique, however, is not known in fluoroalkyl alcohol oxidation using nitric acid.